Problem here is the collisions algorithm, it may require large computing times specially using higher densities. Check your density settings (Distribution Map->X/Y Size), usually you will get the same result lowering it, when most of items are discarded (check General->Forest stats for details).
Anyway, usually it's not necessary to use collisions, specially to create grass. A simple setting of "Full" distribution map, and Random Translation to avoid the grid effect, should be enough. If you need to exclude other items for the grass, use the
Areas features.